A Single-Year Cosmic Ray Event at 5410 BCE Registered in 14C of Tree Rings.

F Miyake,A J T Jull,L Wacker,M Oinonen,R Muscheler,M Salzer,S Helama,F Adolphi,N Brehm,F Tokanai,M Takeyama,K Nicolussi,T Moriya,I P Panyushkina,K Kanzawa

doi:10.1029/2021gl093419

Abstract

The annual 14C data in tree rings is an outstanding proxy for uncovering extreme solar energetic particle (SEP) events in the past. Signatures of extreme SEP events have been reported in 774/775 CE, 992/993 CE, and ∼660 BCE. Here, we report another rapid increase of 14C concentration in tree rings from California, Switzerland, and Finland around 5410 BCE. These 14C data series show a significant increase of ∼6‰ in 5411–5410 BCE. The signature of 14C variation is very similar to the confirmed three SEP events and points to an extreme short‐term flux of cosmic ray radiation into the atmosphere. The rapid 14C increase in 5411/5410 BCE rings occurred during a period of high solar activity and 60 years after a grand 14C excursion during 5481–5471 BCE. The similarity of our 14C data to previous events suggests that the origin of the 5410 BCE event is an extreme SEP event.

Highlights

The cosmic-ray flux into the Earth atmosphere fluctuates over time due to various factors
Prior to direct observations of cosmic rays, signatures of past extreme solar energetic particle (SEP) events can be recorded by cosmogenic nuclides, in particular, 14C in tree rings, and 10Be and 36Cl in polar ice cores, all of which are produced in a particle cascade triggered by interactions of high energy cosmic rays with the constituents of Earth's lower atmosphere (Mekhaldi et al, 2015; Miyake, Usoskin, & Poluianov, 2019)
Büntgen et al (2018) reported an approximately 3‰ and 2‰ offsets observed between the atmospheric radiocarbon zones NH0 and NH2, and NH0 and NH1, respectively (NH0, NH1, and NH2 correspond to the geographical locations of our Finnish, Switzerland, and Californian specimens), and the range of our offsets has a close similarity to the reported offsets

Summary

Introduction

The cosmic-ray flux into the Earth atmosphere fluctuates over time due to various factors. Solar and geomagnetic modulation of galactic cosmic rays are dominating this process on timescales from a few-days to millions of years, such as Forbush decreases (a shielding of galactic cosmic rays due to a coronal mass ejection), the 11-year Schwabe cycle, multidecadal variation such as Grand Solar Minima, or millennial variations in Earth's magnetic field strength. In addition to these recurring factors, sporadic and intense solar energetic particle (SEP) events can contribute to a sudden ejection of cosmic rays to the Earth. Prior to direct observations of cosmic rays, signatures of past extreme SEP events can be recorded by cosmogenic nuclides, in particular, 14C in tree rings, and 10Be and 36Cl in polar ice cores, all of which are produced in a particle cascade triggered by interactions of high energy cosmic rays with the constituents of Earth's lower atmosphere (Mekhaldi et al, 2015; Miyake, Usoskin, & Poluianov, 2019)

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